1) Field of the Invention
The present invention relates to a scanning and imaging optical system, an optical scanner, and an image forming apparatus.
2) Description of the Related Art
Optical scanners associated with image forming apparatuses, such as a laser printer and a digital copying machine, perform optical scanning with an optical deflector and a scanning and imaging optical system. The optical deflector deflect beams of light from a light source, and the scanning and imaging optical system condenses the beams deflected toward a surface to be scanned to form a light spot on the surface. Thus, the optical scanning is performed. Recently, as the scanning and imaging optical system or a part of the scanning and imaging optical system, a resin lens that is manufactured by molding plastic is generally used. The resin lens can be easily mass-produced at a low production cost. Therefore, if the resin lens is applied to the scanning and imaging optical system or a part of the scanning and imaging optical system, manufacturing cost for the optical scanner and the image forming apparatus can be effectively reduced.
Furthermore, with the resin lens, a special lens surface, such as an aspheric surface, can be easily obtained. Therefore, the resin lens largely contributes to a simplified structure of the scanning and imaging optical system by reducing the number of lenses and to improved optical performance.
However, the resin lens has a disadvantage in which refractive index distribution occurs inside the resin lens. In plastic molding, a thermally melted plastic is injected into a metallic mold and cooled down in the metallic mold. The plastic inside the metallic mold is cooled down gradually from a portion touching a surface of the metallic mold. A portion of the plastic located near a core of the metallic mold is cooled down relatively slowly. Resin solidifies and contracts when cooled down, and a volume of the resin decreases. Therefore, the portion cooled down earlier, which is the portion to be a periphery of the resin lens, solidifies before the portion to be cooled down later solidifies. If there is the portion not yet solidified, which has higher temperature and fluidity, in the metallic mold while a part of the plastic is solidified, the portion not yet solidified moves toward the portion solidified, of which the volume has decreased. As a result, the portion to be cooled down later becomes a solid having lower density when solidified.
Thus, in a cooling process in the plastic molding, nonuniform distribution occurs in density inside the resin lens. Since a refractive index of resin is proportional with density of resin, nonuniformity in the density leads to nonuniformity in the refractive index inside the resin lens. In the resin lens, since the density is higher at the portion near the surface than the portion inside the resin lens away from the surface, the refractive index is lowest at an inner deepest portion of the resin lens and gradually becomes higher toward the surface.
Nonuniformity in the refractive index also depends on a shape of the resin lens. For example, in a resin lens having a double-convex cross-section, a thickness of the resin lens decreases from a portion around an optical axis toward a rim of the resin lens. In the cooling process at manufacturing such resin lens, the portion near the optical axis takes more time to be cooled down than a portion near the rim. In such a case, the refractive index is higher at the portion near the rim and is lower at the portion around the optical axis.
In contrast, in a resin lens having a double-concave cross-section, the thickness of the resin lens increases from the portion around the optical axis toward the portion near a rim of the resin lens. Therefore, the portion near the rim takes more time to be cooled down than the portion around the optical axis. As a result, the refractive index is higher at the portion around the optical axis and is lower at the portion near the rim.
Since design of the resin lens is carried out assuming a uniform refractive index, if the refractive index is nonuniform in the resin lens manufactured, performance expected at a designing stage cannot be obtained. When the resin lens is used as the scanning and imaging optical system or a part thereof, an imaging position of the deflected beams is deviated from a position designed, and a field curvature deteriorates, resulting in an increase in a spot diameter of the light spot.
If a resin lens is molded in such a manner that resin in the metallic mold is put in a thermostatic chamber to be cooled down over a sufficiently long time, for example, more than 10 hours, the refractive index distribution inside the resin lens can be made substantially uniform. However, such method decreases productivity of the resin lens, and increases the manufacturing cost. As a result, low cost, which is supposed to be a merit of the resin lens, cannot be realized.
An optical scanner with excellent performance that is realized by considering the refractive index distribution inside a resin lens is disclosed in Japanese Patent Application Laid-Open No. 2003-344756. Furthermore, a tolerance of the resin lens used in an optical scanner in the refractive index distribution is disclosed in Japanese Patent Application Laid-Open No. 2000-352679. A method for measuring the refractive index distribution inside a resin lens is described in Japanese Patent Application Laid-Open No. H11-044641.